This invention relates to the field of fiber optic sensor systems, and more particularly to the field of compensating such systems for fiber optic lead and connector losses and power fluctuations of the optical source.
Fiber optic technology has been growing at an ever expanding rate. With the advance in fiber optic technology, the adaptation of optical devices to sensing systems is becoming more widespread. For example, fiber optic sensors have been proposed to detect such phenomena as acoustic waves, rotation rates, acceleration, pressure, magnetic and electrical fields, temperature, and stress and strain, etc. However, a problem inherent in all such fiber optic sensing systems is that the optical signal is greatly attenuated by the fiber optical cable itself and its connectors. In addition, power fluctuations in the optical source may also obscure many usable signals from such a device. Thus, detecting and precisely measuring an applied pheomenon is very difficult, severely restrictng the uses to which a fiber optic sensor may be applied.
FIG. 1 depicts the relationship between detected optical power and an applied phenomenon (stress) in known fiber optic sensor systems. Where the detected light intensity is I.sub.s, it is unknown whether this detected intensity corresponds to a stress level S.sub.1, S.sub.2, or S.sub.3. This is because the amount of lead and connector losses in the fiber optic cable system may vary from slight to extreme. The detected signal is difficult to correlate with a given stress level. Such an uncertainty is often fatal to a proper application of such fiber optic sensor systems.
One solution to this problem is proposed in U.S. Pat. No. 4,368,645 to Glenn et al. Glenn et al discloses an optical pressure sensor which can compensate for some (but not all) lead/connector losses and light source power fluctuations. The device according to Glenn et al provides a light source which generates a light beam that is input into a fiber optic cable. The fiber optic cable directs the source light beam to a lens which collimates the light beam. The collimated light beam is then passed through a linear polarizer and a quarter wave plate to circularly polarize the collimated light beam. The circularly polarized light beam is then introduced into a photo transducer which modulates the polarized light beam in accordance with pressure applied to the photo transducer. The modulated light beam is then directed to a polarizing beam splitter which splits the modulated light beam into first and second components. Each separate component is focused by a lens into a separate fiber optic cable. These fiber optic cables then direct the first and second components to photo detector devices for detecting the intensity of the first and second components. The dependence of the intensities of the two components on the pressure applied to the photoelastic transducer permits the measurement of that pressure in a manner that has quadratic error dependence on optical misalignment. The difference in the intensities of the two components is then divided by the sum of the intensities of the two components to eliminate lead/connector losses in the fiber optic cable leading up to the transducer. However, such a scheme does nothing to accommodate for lead/connector losses from the output of the transducer to the photo detector devices. Thus, the device according to Glenn et al is still highly susceptible to fiber optic lead/connector losses. If such losses in the output fiber optic cables are significant, the device according to Glenn et al will not function properly.
Another solution for compensating the sensitivity variations in fiber optic cable damping and the drift of the light source and photo detectors is proposed in U.S. Pat. No. 4,493,995 to Adolfsson et al. In Adolfsson et al, an optical source provides a source light beam at a single wavelength which is used as a carrier wave. This carrier wave is then modulated at one or more lower frequencies in accordance with the phenomenon sensed by the photo transducer. The material in the photo transducer responds differently depending upon the modulation frequency of the light beam, not upon the carrier frequency (wavelength) itself. The response of the photo transducer material at the different modulation frequencies can then be used to determine the sensed phenomenon independently of lead and connector losses. However, such a device is complex and expensive due to the necessity of modulating the carrier wave. In addition, demodulation electronics are required, also increasing the complexity and cost of this device.
Therefore, what is needed is an inexpensive yet precise solution to compensating photo sensor systems for fiber optic lead/connector losses and fluctuations in the light source.